1. Field of the Invention
Embodiments of the present invention relate generally to an electron beam writing apparatus and a method for adjusting an electron beam convergence half angle. More specifically, for example, the embodiments relate to a method for adjusting a convergence half angle of an electron beam in the electron beam writing apparatus that emits an electron beam onto a target object.
2. Description of Related Art
In recent years, with high integration of LSI, the line width (critical dimension) required for circuits of semiconductor devices is becoming progressively narrower. As a method for forming an exposure mask (also called reticle) used to form circuit patterns in these semiconductor devices, the electron beam (EB) writing technique having excellent resolution is employed.
FIG. 10 is a conceptual diagram explaining operations, of a variable shaped electron beam writing or “drawing” apparatus. The variable shaped electron beam (EB) writing apparatus operates as described below. A first aperture plate 410 has a quadrangular aperture 411 for shaping an electron beam 330. A second aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the aperture 411 is deflected by a deflector to pass through a part of the variable shape aperture 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during writing. In other words, a quadrangular shape that can pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) system.
In electron beam writing, importance is attached to the throughput, in manufacturing a mask. On the other hand, importance is attached to the resolution of beams, in performing various evaluation to develop next-generation lithography, because finer pattern formation is requested therefor.
In terms of attaching importance to the throughput, if luminance of the cathode of an electron gun is increased, cathode conditions such as a crossover radius of an electron beam immediately after discharge of the cathode may change, resulting in degradation of resolution.
On the other hand, the resolution depends upon the electron optical aberration, and the electron optical aberration is proportional to the power (exponentiation) of the convergence half angle. The convergence half angle depends upon the crossover radius of an electron beam. Therefore, in terms of attaching importance to the resolution, in order to reduce a convergence half angle, it is examined to decrease the crossover radius of a beam by focusing electron beams emitted from the electron gun while strengthening the excitation of the illumination lens.
Then, although it is necessary to increase the excitation of the illumination lens as much as possible for reducing the convergence half angle, there is a limit in increasing a magnetic flux density due to magnetic saturation, etc. of pole piece material used for the illumination lens. Therefore, there is also a limit in increasing the excitation of the illumination lens in accordance with the limit of the magnetic flux density. Accordingly, there is also a limit in focusing an electron beam by controlling the illumination lens, and thus, there is also a limit in reducing the convergence half angle. As a result, there is a problem in that resolution needed for development of next-generation lithography may not be acquired. 7
Furthermore, if a convergence half angle is made small, the current density of a beam is also made small. Then, if the current density is decreased, writing time needs to be long, and thus resulting in a problem that the throughput degrades.
Regarding this, there is disclosed a technique for changing beam crossover characteristics by using a plurality of accelerating electrodes (refer to, e.g., Japanese Published Unexamined Patent Application (JP-A) No. 2000-182550).